Abstract: Although supernovae are rare nearby, they are not rare in the universe, with a total rate exceeding one per second. The neutrino flux from all past core-collapse supernovae, while very faint, is realistically within reach of detection in Super-Kamiokande. I will discuss how predictions for this flux are supported by recent data on the cosmic star formation rate, the prospects for neutrino detection, and the implications for understanding core-collapse supernovae. Finally, I will show that the present neutrino flux limit implies a stringent limit on the contribution of type Ia supernovae to the diffuse MeV gamma-ray background; this poorly understood background is often used as a constraint on new particle physics models.